Noise cancellation circuit



May 21, 1963 E. c. FLOYD 3,090,832

NOISE CANCELLATION CIRCUIT Filed Jan. 30, 1961 2 Sheets-Sheet 1 ATTY.

May 21, 1963 E. c. FLoYD NoIsE CANCELLATION CIRCUIT 2 Sheets-Sheet 2Filed Jan. 30, 1951 wm IVR W1 +m .E s3 MQ EN wh +m INI/Enron. ra/5T/0907 er fwn my.

United States Patent O 3,090,832 NGISE CANCELLATION CIRCUIT Earl C.Floyd, Westchester, lll., assignor to Admiral Corporation, Chicago,lll., a corporation of Delaware Filed Jan. 30, 1961, Ser. No. 85,614 3Claims. (Cl. 178-7.3)

This invention relates in general to television receivers and inparticular to circuitry therein for improving the visual displaypresented on the screen of a television receiver under adverse signalconditions.

A conventional television signal includes video information componentsfor reproducing a visual indication of the scene being televised, audioinformation components for reproducing the sound accompaniment andsynchronizing components for controlling operation of the horizontal andvertical scanning circuits in the television receiver. The videoinformation and synchronizing components of the television signalcomprise amplitude modulations, whereas the audio components comprisefrequency modulations. The synchronizing components are separated inamplitude from the video information components, being larger than anyof the video information components and occurring at relatively constantamplitude levels. The television signal is subject to noise pickup frommany sources and often the noise components greatly exceed the magnitudeof the synchronizing components.

These noise signals often interfere with thev proper synchronization ofthe circuitry in the television receiver responsible for developing thehorizontal and vertical sweep voltages. 'Ihe results of this type ofinterference may be picture jitter, horizontal tearing, verticalrolling, etc. As is well known in the art, the vertical circuit is mostsusceptible to disturbances of this type.

Most well designed television receivers have some sort of noiseprotection circuitry. In general there are two main methods of noiseprotection. One is noise gating and the other is noise cancellation. Inthe noise gating type of noise protection circuit, noise pulses above acertain amplitude are effective to immobilize the synchronizing signalseparator circuit in the television receiver. The

theory is that it is better to lose output from the synchro- I nizingsignal separator for a brief interval of time, usually corresponding toa few horizontal lines, than to allow large noise pulses therein whichmay prematurely trigger the circuits responsible for developing thedeflection voltages.

The noise cancellation technique is different in that it is based on theprinciple that the noise signal may beremoved from the television signalby algebraically combining it with a similar though oppositely poledsignal developed responsive to the noise signal. This systemhas theadvantage of insuring that the synchronizing circuits of a televisionreceiver will not be immobilizedeven for short periods of time, but willalways remain operational.

While the theory of noise cancellation has been known for some years,satisfactory results with circuits embodying this theory have not beenobtainable. All of the circuits of the prior art have sulered fromvarious defects, since it is a diicult task to develop a noisecancelling circuit Which will respond faithfully to the extremeVariations in signal level and signal to noise ratios encountered in theoperation of a modern day television receiver. The circuit of theinvention embodies a combination of elements cooperating in a mannersuch that the noise cancellation circuit operates faithfully overextremely wide variations in signal level and signal to noise ratio. Oneof the features of this circuit, aside from its excellent performance,lie's in the use of a single tube envelope housing three independenttriode tube sections, respectively performing noise inversion andcancellation, synchronizing signal seperation, and automatic gaincontrol voltage development.

HCC

Accordingly the principal object of this invention is to provide animproved noise protection circuit for television receivers;

Another object of this invention is to provide proved noise cancellationcircuit;

Still another object of this invention is to provide a noisecancellation circuit employing a noise inverter tube in which theconduction level of the noise inverter tube is continuously adjusted inaccordance with signal level changes; Y

A further object of this invention is to provide a noise cancellationcircuit for television receivers which not only performs better thansimilar circuits of the prior art but does so in a more economicalmanner;

A still further object of this invention is to provide a noisecancellation circuit for a television receiver in which the videoamplifier output capabilities and the noise inverter cancellationability are so apportioned as to prevent over cancellation of noisepulses during weak signal reception;

A feature of this invention resides in the provision of a single tubeenvelope incorporating therein three independent triode tube sections,one for noise inversion and cancellation, another for synchronizingsignal separation, and a third for development of an automatic gaincontrol potential.

Another feature of this invention lies in the provision of meansinterconnecting the screen electrode of the video amplifier and the gridof the automatic gain control tube whereby the automatic gain controlsection of the receiver is assisted in operation for both extremelystrong and eX- tremely Weak input signals to the video amplifier.

Further objects and features of this invention will be apparent uponreading of the specification in conjunction with the drawings in which:

lFIG. 1 represents a block diagram of a complete television receiverembodying the circuit of the invention; land FIG. 2 represents aschematic diagram of the circuit of 'the invention.

Referring now to FIG. 1, a television signal is received by antenna 5and coupled to converter 6. Converter 6 is well known in the art and'includes a radio frequency amplifier, a local oscillator, and a mixerfor developing an intermediate frequency television signal. The outputof converter 6 is coupled to an intermediate frequency an im- 'amplitier7 where the intermediate frequency signal is amplified. The intermediatefrequency (IF) signal is fed to video detector 8 which drives videoamplifier 10. Y

An audio circuit 9 is shown coupled to video detector 8 and includes asound intermediate frequency amplifier, a sound detector, a soundamplifier and a speaker. Audio circuit 9 may be coupled instead to thevideo amplifier output by use of suitable traps, as is known in the art.Video amplifier ltl'amplies the composite video signal from videodetector 8 and is coupled to a synchronizing signal separator l1 and anautomatic gain control (AGC) circuit 17. `Video amplifier 10 also drivesthe signal input circuit (not shown) of picture tube 13. A portion ofthe video detector signal is coupled to noise inverter 14, which is alsocoupled to video amplifier 10. Noise inverterv 14 operates, in a mannerto be described Afully hereinafter, to sense large noise impulses in thesignal from video detector 8 and to develop corresponding, thoughoppositely poled, signals for application to the output of videoamplifier 10. Noise cancellation occurs and synchronizing signalseparator (sync separator) 11 is fed a relatively noise free signal.

The output of sync separator 11 is coupled to the vertical circuit 12which is responsible for generating deflection voltages in conjunctionwith the vertical Winding on picture tube 13. The sync separator is alsocoul 3 pled to the horizontal and AFC (automatic frequency control)circuit 15.

The horizontal oscillator (not shown) is of the free running type andhas a natural frequency of oscillation close to the horizontal linesynchronizing frequency of the television signal. The output of theoscillator feeds a horizontal outputV circuit (-noty shown) which iscoupled to the receiver high voltage section 176. The AFC circuitcompares the horizontal oscillator frequency with the frequency of thehorizontal line synchronizing componentsin the television signal andprovides correction voltages to hold the horizontal oscillator at theline synchronizing frequency. This arrangement is well Iknow-n in theart.

High voltage section 16 develops the horizontal sweep voltages for thedeection windings of picture tube 13 as well as the high voltage directcurrent necessary for operation of the picture tube. High voltagepulses, occurring at the horizontal. sweep rate, are, fed to AGC circuit17 to develop what is commonly called a gated or keyed AGC controlvoltage. High voltage section 16 is also coupled to horizontal and AFC'circuit 15 to provide the necessary pulses for the automatic frequency'control' section to stabilize the horizontal oscillator at the properfrequency.

The AGC voltage developed in AGC circuit 17 Vis coupled to the converter6' and also to the IF amplifier 7. Noise inverter circuit 14 is :coupledto the I-F amplier AGC potential. IIt should be noted that the leadconnecting video amplifier with AGC circuit 17 actually comprises twoindividual coupling paths as will be apparent by reference to FIG. 2.

' Winding 20 is tuned lby parallelly connected capacitor 21 andcomprises a portion of the ylast intermediate frequency transformer (notshown) in the intermediate .frequency amplifier 7 of FIG. 1. Winding 20is connected to ground at its lower terminal and to a diode detector 22at its upper terminal. A series of .tuning coilsV 23, 24 and 25 areutilized in a conventional manner so that a negative -going compositevideo signal is coupled -to control grid 33 `of video amplifier tube 30.This composite video signal is developed across the cir'- cuitcomprising winding 25 and tapped resistor 26. Tapped -resistor 26 isrequired since the video detector must also 4feed the cathode of thenoise inverter tube, which constitutes a heavy load. Therefore only aportion of the composite video signal is coupled to the cathode byvirtue of tapped resistor 126.

Video amplifier 30 comprises a pentode type tube having an anode 31, acathode 32, a control grid 33 and a screen grid 34. The last grid, thatis the suppressor lgrid, is not indicated with ka separate referencenumeral since it plays 4no part in the invention and is assumed to beconnected in a conventional manner. Cathode 32 is grounded and anode 31is connected to B-I- through a tuned circuit comprising capacitor 28 andcoil 27, a peaking coil 29, and a load resistance combination. 'llheload resistance combination comprises resistor 35 which is parallelledby serially connected resistors 36 and 37. A resistor 38 is connected tothe junction of resistors 36 and 37 andthe anode of noise tube 40.B-lfor screen grid 34 is supplied through -a fairly large resistor 80. Acapacitor 81 bypasses the screen grid to ground.`

NoiseV tube 40 includes `an anode l41, a cathode 42 and a control grid43. Cathode 42 is connected to tapped resistor 26 which is used toselect a portion of the composite video signal. Control grid 43 isconnected to a junction 44. A capacitor 39' couples the anode of noisetube 40 to sync -separator tube 50. The entire coupling circuitcomprises capacitor 39 and resistor 45, to the junction of which isconnected the parallel combination of resistor 46 and capacitor "47. Theother end of this combination of resistor 46 and capacitor 47 isconnected to gridr 53 of sync separator tube 50. This arrangement iswell known in the 4art and is commonly called a double time constantcoupling circuit for the sync separator tube. Anode 51 of ysyncseparator 50* is coupled to a low 'I3-{- supply voltage through a loadresistor 48. Cathode 52 of tube 50 is connected to ground. A resistor 49is connected to the junction of anode 51 and resistor `48 and inconjunction with resistor 48 comprises a voltage divider arrangement forthe sync separator tube. Anode 51 is coupled to horizontal cir-` cuitry(not shown) through a suitable coupling capacitor 55. Y

The junction of video amplifier load resistors 35 and `37 is connectedto grid 73 of AGC tube 70 through a resistor 60. Grid 73 is connected,through a xed resistor 61 and a variable resistor 62, to ground.Variable resistor `62 is labelled AGC Set and is utilized to adjust theautomatic gain control level for the individual television receiver inaccordance with the strongest sig- ,nal which said receiver is toreceive. Another connection -to grid 73 exists from B+, through resistor56, through variable -resistor v57, and through a fixed resistor 59. Alower portion of variable resistor '57, which is the contrast control inthe receiver, is bypassed by a .capacitor 5S. Additionally, the upperterminals of resistors 59 and i6() are joined, this junction also beingconnected to the video amplifier lload resistors. A connection to grid73 also exists, through a resistor 63, Vto the screen grid 324 of videoamplifier 30. v

AGC tube 70 has its anode 71 coupled through a capacitor 74 to highVolta-ge section 16 (not shown in this igure). Anode 71 is alsoconnected to a pair of inte- -grating networks comprising resistor 75and capacitor 76, and resistor 7S and capacitor 79, respectively. Thejunction between 4resistor 78 and capacitor 79, llabelled RF-AGC, isconnected to converter `6, shown in FIG. -1. The junction of resistor 75and capacitor 76, labelled IF-AGC, is connected to IF amplifier 7 inFIG. 1. This latter junction feeds a network comprising resistor 77,resistor 68, and capacitor 69, lwhich is coupled to junction 44,previously mentioned. Junction 44 V is also connected, through aresistor l65, to a Noise Set variable resistance control'166. One end ofthis control is connected to ground through a resistor 67, and the otherend to the grid of the horizontal output tube (not shown) in horizontaland AFC circuit 15 of FIG; l.

The latter connection is through a resistor 64. A description of theoperation of the circuit now follows.

=Omitting the function of the noise tube 40 and the 4audio circuit, thetelevision intermediate frequency signal is detected by video detector22, which develops the composite video signal. This signal is fed tocontrol grid 33 of video ampliiier 30. As is indicated by diagram 100,this composite signal is negative going and may contain Vspurious noisepulses of relatively large amplitude. The signal is amplified by videoamplifier 30 and coupled to sync separator 50 Iand AGC tube 70. Thedouble `time constant input circuit of sync separator 50 acts to passonly the extreme portions of the signal, that is the synchronizingcomponents. These components are separated in sync separator 50 andcoupled to the horizontal and vertical circuits, respectively. 'Iheamplified composite video signal is'coupled to .the grid of the AGC'tube 70, the anode of which is subjected to a gating or keying pulse,indicated by waveform 100. This type AGC circuit is well known in theart and is commonly called gated or keyed AGC. 'Ilre high voltage pulsesof waveform 100 occur in synchronism withV the synchronizing componentsof the signal fed to grid 73. The cathode 72 of AGC tube 70 is connectedto a source of low B+ and hence the Vtube is normally cut-off. However,during the occurrence of synchronizing components, a high voltage pulseis fed to Ithe anode 71 of AGC 'tu-be 70, driving the tube conduc-tive.The magnitude of the conduction current drawn -by AGC tube 70 isdetermined by the magnitude of the synchronizing components in thecomposite video signal. This circuit is elective to produce negativepotentials at the points labelled IF-AGC and RF-AGC which potentialsvary in accordance with the magnitude of the incoming signal.

If the composite signal is large, grid 73 is driven heavily .positiveand a large current .flows through tube 70, resulting in a largenegative control voltage being developed across the integratingnetworks. Conversely, if Ithe signal is small the opposite eiect -occursand a correspondingly smaller control voltage is developed. Thedeveloped AGC potentials are fed back to converter 6 and IF amplifier 7to control :the gain of these .portions of the television receiver.Thus, under normal signal conditions, the magnitude of the input signalto video detector 22 is held relatively constant as a result of this AGCaction.

A portion of the composite video signal is coupled to cathode 42 ofnoise inverter tube 4t). This signal is negative going and tends :todrive the noise tube conductive. However, noise tu-be 40 is maintainedat cut-oilr by virtue of the negative potential impressed on grid 43,from the .grid circuit of the horizontal output tube (not shown). Thisnegative potential is substantially constant for all signal conditions.

v Grid 43 of noise tube 41) thus has a constant negative potentialapplied thereto to maintain it nonconductive for all video andsynchronizing information components of the composite television signal.Grid 43 is also connested through a network previously described to thelF-AGC control potential which varies in accordance with signal level.The Noise Set control 66 is adjusted so that the potential on grid 43 issutiicient to hold noise tube 50 nonconductive for all negative signalsapplied to cathode 42 except those a predetermined amount or more inexcess of the amplitude of the synchronizing portions of the compositetelevision signal. Upon occurrence of such large signals, noise tube 40is driven conductive as cathode 42 becomes negative with respect to grid43. Assuming a waveform substantially as shown at 100, the Waveformapplied :to cathode 42 Will be as shown at 102. However, only theportions below dashed line 102 will be effective to cause conduction intube 40.

Waveform 101 indicates the amplied composite video signal and the noisepulses therein. For normal signal levels to video ampliiier 30, themaximum amplitude of noise pulses in the video amplifier output islimited by cutoi of the tube. Noise pulse in excess of those causingcutoff in the grid circuit are clipped. The output signal trom videolamplilier 30 is algebraically added across resistor 38 to the outputsignal of noise tube 40, resulting in cancellation of the noise pulses.The result is a waveform substantially as shown by 193. This noise freewaveform is coupled by the coupling network previously indicated to grid53 of sync separator 50 where the synchronizing components areseparated. Thus, the noise pulses are prevented from entering thesynchronizing signal separator circuit and hence can produce nodeleterious eiects upon the horizontal and vertical circuitry of thetelevision receiver.

It should be obvious .to those skilled in the art that for normalsignals, a xed Noise Set level will suice to cancel noise pulses beforethey reach the synchronizing signal separator. However, in the presenceof large variations in signal strength a fixed noise tube bias isundesirable. If the signal is excessively strong, noise tube 40 may bedriven conductive responsive tothe synchronizing components and resultin cancellation of these components. This of course will result incomplete loss of synchronization in the television receiver. If thesignal is weak and noisy, the signal to noise ratio will be decreased by.the video amplier -gain characteristic and result in incomplete, ifany, noise cancellation by noise tube 40.

This situation is obviated by connecting noise inverter grid 43 to anadditional control source which is capable of following variations inthe levels of the received television signals. Connection of Igrid 43through the coupling circuit to the IF-AGC potential provides thisvariable potential.

Ideally, noise tube 40 should have a large ampliiication factor as wellas a high plate current capability. However, triode tubes having thesecharacteristics are not generally available and therefore means areprovided :to match the capabilities of the video amplifier tube with thecapabilities of the noise inventer tube. This means comprises circuitryfor coupling the output of the video amplifier tube to the noise tubeanode at a reduced level. Ihe combination of elements performing thisfunction are resistors 35, 36 and 37.

In this type circuit it is highly desirable that the AGC control voltagetrack the incoming signal accurately. As is Well known in the art, whena rapid transition is made from a strong signal to a weak signal or viceversa, both the AGC circuit 'and the sync separator circuit may notrespond quickly enough and the result may be missynchronization in thetelevision receiver. To assist the sync separator circuit, andato alimited extent -the AGC circuit, a circuit described in Patent No.2,927,156-, issued to Robert Jones, is u-tilized. Briefly, theconnection of screen grid 34 of video amplier tube 30 to B+ rthroughresistor helps keep the signal to the sync separator near the value towhich it is held when the -AGC circuit is effective. The screen griddraws current in proportion to the conduction'current flowing in thevideo ampliiier. -If an extremely weak signal is fed to the controlkgrid of the video ampliiier the conduction current therein increases.This increase 'in conduction current is ysensed by the screen `gridwhich, due to its connection to B-fthrough resistor 80, suffers =avoltage drop. Thus the screen grid potential lfalls as it draws morecurrent. This ldecline in screen grid potential in effect shifts thetransfer characteristic of the video arnplier tube in a direction whichtends to maintain the level of the sync tips in the output signal at thelevel to which they are normally held by fthe AGC circuit. Thus the syncseparator 4is protected from large, rapid changes in input signal level.By maintaining the input signal level to the sync 4separator iixed, the-arnplitude of any noise -fed thereto is minimized. Thus during thiscritical transient interval the noise inverter is also assisting inperforming its cancellation lfunction.

Lt will be recalled that the screen ygrid of they video ampliiier isalso connected to the control grid of the AGC tube .through a resistor`63. This connection greatly enhances the response of the AGC circuit,which circuit controls the conduction level of the noise inverter tube.Assume va'large input signal to the video ampli; tier. The videoarnpliiier will be cut oi lby the sync components and the screen gridwill cease drawing current. This results in a rapid increase in thescreen voltage, which increase is immediately reflected at the AGC rtubecontrol grid, thus insuring rapid development of AGC voltage. This AGCvoltage reduces the gain of the receiver prior to the video detector andbiases the noise inverter tube lback strongly to prevent cancellation'ofthel sync pulses. Conversely, upon receipt of a very Weak signal (largeconduction current in the video amplifier), the screen voltage fallsquickly and this change is communicated Ito the AGC tube which resultsin a smaller developed control voltage. Hence the noise inverter canrespond quickly to rapid signal level changes, thus greatly improvingits noise cancelling -ability in these critical areas.

The connection of the control grid ofthe noise tube to a source ofcontrol potential which varies in accordance with the signal level alsoprevents over cancellation of noise pulses in the video signal whichlgive rise to white spots in the displayed picture. These white spotsare the result of holes being cut into the video information when thecancellation pulse is larger than Ithe noise pulse. Over cancellationoccurs in the presence of extremely strong signals in which the -synctips come very close to the cut-oit portion of the Video amplifiertransfer characteristic, kIn such case, noise pulses extending Ibeyondthe sync tips are clipped and hence are not materially iarger, in theoutput circuit of the video amplifier, than the sync'ltip portions.These noise pulses however are not clipped or compressed when they arecoupled to the cathode of the noise tube, land give rise to largecancellation signals. The algebraic cancellation of the positive goingnoise pulse with the negative Igoing cancellation pulse at the anode ofthe. noise tube may result in over cancellation and consequent choppinginto the video portion of the signal. This effect is substantiallyeliminated by adjusting the noise tube conduction level in accordancewith the incoming signal strength so that Y'for strong signals the noisetube is much less sensitive, that is it is more di-cuflt to drive intoconduction.

Ina practical circuit embodying the invention tubes 40, 50 and 70 areall contained in one tube envelope. This tube -arrangement makes for avery economical package of excellent perfomance. The Noise Set controlis not required lonce the circuit parameters have been established, itbeing replaced Iby iiXed resistors. The circuit as [described utilizesIkeyed AGC, but non-gated AGC mayalso be employed.

It is understood 'that numerous modifications in the circuits shown maybe made without departing from the true spirit and scope of theinvention as defined in the following claims.

What i-s claimed is:

1. In combination in a television receiver including means for producing1a composite video signal having information components and periodicreference components from a received television signal, said referencecomponents being greater in magnitude than said information components,said television signal being subject to undesired spurious components ofgreater magnitude than said reference components; a video amplifier anda load resistor therefor; means for applying said composite signal -tosaid video amplier; a noise inventer tube Afor inrver-ting andcancelling ysaid spurious components includ-V ing an anode, a cathodeand a control grid; a resistor connected Ibetween a point intermediatethe ends of said video ampliiier load resistor and said anode lforcoupling the output [of said video amplifier to said noise inverter'anode at reduced level; means lfor -coupling said composite videosignal to said cathode at a reduced level; means -for applying aconstant direct current bias to said control grid whereby said noiseinverter tube only conducts on signals of greater magnitude .than saidreference components; control voltage means coupled to said videoamplilier load resistor for developing a control voltage in yaccordancewith the magnitude of said reference components; means in said videoamplifier, and responsive to the conduction current in said videoamplier, coupled to said control voltage means for conveying informationregarding changes in said conduction current thereto; and means forcoupling said control voltage to said control lgrid whereby ftheconduction level of said noise inverter rtube is automatically ladjustedas the level of said composite video signal varies.

2. In combination inV a television receiver including means =forproducing a negatively yoriented composite video signal havinginformation components and periodic reference components from a receivedtelevision signal,

said reference components being greater Vin amplitude than saidinformation components, said television signal being subject to`undesired spurious noise lcomponents of greater amplitude lthan saidreference component-s; a video amplifier including ya cathode, yacontrol grid, a screen grid and an anode; means for applying saidnegatively oriented composite video signal between the cathode andcontrol grid of said Yvideo amplifier; a source of positive potential; atapped load resistance'connected between said video amplifier anode 'andsaid source of potential; a noise inverter tube for inverting andcancelling said spurious noise components including an Ianode, a cathodeand a control grid; a resistor connected lbetween the tap on said videoamplifier load resistance and the anode of said noise inverter tube forcoupling the output of said video amplifier at reduced level to saidnoise inverter anode; direct current means coupling said negative goingcomposite video signal to the cathode of said noise inverter tube atreduced level; means for applying a constant negative direct currentbias portential to the control grid of said noise inverter tube wherebysaid noise inverter tube is. only driven conductive for signals ofgreater amplitude than said reference components; separating meanscoupled tothe anode of Isaid noise inverter tube yfor separating saidperiodic reference component-s; a gain control tube having a cathode, acontrol `grid and an anode; direct current means yfor coupling said loadresistance of said video 4ampliiier to the control grid of said gaincontrol tube; an integrating network connected -to ,the anode of saidgain control tube; means for applying high Voltage pulses fto the anodeof said gain control tube in synchronism with said periodic referencecomponents on its control grid for driving said tube conductive -inaccordance with the amplitude of said reference components,v saidintegrating net- =work developing a negative control potential varyingas :a function of the amplitude of said reference components; means,including a resistor, coupling the screen grid electrode of said videoampliiier to said source of potential; means connecting the screen gridelectrode of said video amplifier to the control grid of said gaincontrol ftube for coupling variations in the videoampliter conductioncurrent to the control grid of said gain control tube; means couplingsaid integrating network to the control Igrid of said noise invertertube for automatically yadjusting the conduction level .thereof `inaccordance with signal level changes, whereby said noise inverter tubeis driven conductive by spurious noise components ot greater amplitudethan said reference components and cancels said spurious noisecomponents present in the output of said video ampliiier tube thusproducing a signal free of said spurious noise components for saidseparating means. t

3. In a television receiver as set forth in claim 2 wherein said noiseinverter tube, said separating means, and said gain control tube,individually comprise independent electron discharge devices included ina common evacuated tube envelope.

References Cited in the ytile of this patent UNITED STATES PATENTSGibson Aug. 23, 1960

1. IN COMBINATION IN A TELEVISION RECEIVER INCLUDING MEANS FOR PRODUCINGA COMPOSITE VIDEO SIGNALS HAVING INFORMATION COMPONENTS AND PERIODICREFERENCE COMPONENTS FROM A RECEIVED TELEVISION SIGNALS, SAID REFERENCECOMPONENS BEING GREATER IN MAGNITUDE THAN SAID INFORMATION COMPONENTS,SAID TELEVISION SIGNAL BEING SUBJECT TO UNDESIRED SPURIOUS COMPONENTS OFGRETER MAGNIUTDE THAN SAID REFERENCE COMPONENTS; A VIDEO AMPLIFIER AND ALOAD RESISTOR THEREFOR; MEANS FOR APPLYING SAID COMPOSITE SIGNAL TO SAIDVIDEO AMPLIFIER; A NOISE INVERTER TUBE FOR INVERTING AND CANCELLING SAIDSPURIOUS COMPONENTS INCLUDING AN ANODE, A CATHODE AND A CONTROL RIGID; ARESISTOR CONNECED BETWEEN A POINT INTERMEDIATE THE ENDS OF SAID VIDEOAMPLIFIER LOAD RESISTOR AND SAID ANODE FOR COUPLING THE OUTPUT OF SAIDVIDEO AMPLIFIER TO SAID NOISE INVERTER ANODE AT REDUCED LEVEL; MEANS FORCOUPLING SAID COMPOSITE VIDEO SIGNAL TO SAID CATHODE AT A REDUCED LEVEL;MEANS FOR APPLYING A CONSTANT DIRECT CURRENT BIAS TO SAID CONTROL GRIDWHEREBY SAID NOISE INVERTER TUBE ONLY CONCUCTS ON SIGNALS OF GREATERMAGNITUDE THAN SAID REFERENCE COMPONENTS; CONTROL VOLTAGE MEANS COUPLEDTO SAID VIDEO AMPLIFIER LOAD RESISTOR FOR DEVELOPING A CONTROL VOLTAGEIN ACCORDANCE WITH THE MAGNITUDE OF SAID REFERENCE COMPONENTS; MEANS INSAID VIDEO AMPLIFIER, AND RESPONSIVE TO THE CONDUCTION CURRENT IN SAIDVIDEO AMPLIFIER, COUPLED TO SAID CONTROL VOLTAGE FOR CONVEYINGINFORMATION REGARDING CHANGES IN SAID CONDUCTION CURRENT THERETO; ANDMEANS FOR COUPLING SAID CONTROL VOLTAGE TO SAID CONTROL GRID WHEREBY THECONDUCTION LEVEL OF SAID NOISE INVERTER TUBE IN AUTOMATICALLY ADJUSTEDAS THE LEVEL OF SAID COMPOSITE VIDEO SIGNAL VARIES.